The present invention relates to dry gas seals for industrial compressors, blowers, vacuum pumps, expanders and the like, and in particular to a dry gas shutdown seal.
Dry gas seal systems are used in a wide variety of rotary shaft devices, including blowers, compressors, vacuum pumps, expanders and devices in the turbo machinery industry, all of which have critical sealing requirements. Dry gas seal systems provide a barrier between the gas in the working chamber, or process gas, and the external environment, to minimize the loss of process gas to the environment. These seals are positioned adjacent the interface of the rotating shaft with the stationary working chamber or housing. In general, dry gas seals may be of the circumferential type, or the face type, and are used in pipeline compressors, refineries, utility plants and other similar applications.
Buffer gas, which is normally regulated and filtered compressor discharge gas, is commonly used in dry gas seals to provide a clean seal environment. Used in conjunction with a process side labyrinth seal, buffer gas flows through the seal cavity, then through the labyrinth where it combines with the compressor process gas. A problem that typically occurs in centrifugal compressors is that once the compressor is shutdown, the source of buffer gas is lost. When this occurs, a reverse flow of residual process gas into the seal cavity results. Solid and/or liquid contaminates often contained in the process gas are then deposited on the critical dry seal surfaces. This contamination will eventually effect the performance of the dry gas seal, and shorten the life expectancy of the seal. As the seal life expectancy is decreased, the operating costs increase, because the seal must be replaced more frequently. This replacement requires that the compressor be shutdown more frequently, thereby reducing overall operational efficiency.
One aspect of the present invention is to provide a dry gas seal having a housing with an annularly-shaped channel disposed concentric with the drive shaft, and an inlet to introduce pressurized buffer gas into the housing interior. An annularly-shaped shutdown piston is slidably received in the housing channel for axial shifting, and includes a circumferential seal between its inside and outside surfaces, and an end seal. A biasing member resiliently urges the shutdown piston toward an open position, such that during compression operation, the forces generated by the biasing member combined with the pressure of buffer gas acting on the piston""s interior face retain the shutdown piston in the open position. During compressor shutdown, when the pressure of the buffer gas drops, the pressure of the process gas acting on the piston""s exterior face overcomes the combined forces of the biasing member and the buffer gas to shift the shutdown piston to a closed position against an adjacent sealing surface to prevent contaminates from entering the housing interior.
Another aspect of the present invention prevents contaminated process gas from entering the dry gas seal cavity when the compressor is near or at shutdown. In one embodiment, the shutdown seal system contains four major components that include a split housing with an annular opening, an annular-shaped ring or piston, a segmented circumferential seal ring and a rotating sleeve. The segmented circumferential seal is positioned with springs against the stationary seal housing and hard surfaced sleeve that rotates with the shaft. At low differential pressure, the seal is capable of regulating buffer gas flow into the compressor process, or seal against reverse process gas flow into the seal cavity. During compressor operation, the shutdown piston is held in the normally open (non-sealing) position by multiple fastened springs and gas pressure. The piston is sealed on the inside and outside diameters by fluorocarbon C-seals, and it has limited freedom to slide axially as regulated by spring and pressure forces. In the sealing mode, which occurs during compressor shutdown, the fluorocarbon C-seal mounted in the piston face creates a static seal against the adjacent gas seal shoulder retainer. Under normal operating conditions, filtered buffer gas enters the cavity between the dry gas seal and the circumferential seal. Buffer gas is delivered through a compressor port, which in turn feeds a sealed annular-shaped channel concentric to the compressor shaft and located on the outer diameter of the seal housing. Ports or radial holes in the seal housing direct the buffer gas inwardly from the channel and into the seal cavity. To create flow, the buffer pressure is regulated slightly above the process pressure. This pressure differential is regulated externally, and the circumferential seal acting against the sleeve and housing controls flow. When the compressor is shutdown (i.e., not rotating), the source of high pressure seal gas is lost and the pressure differential across the circumferential seal becomes zero or may actually reverse in direction. With this invention, process contamination of the gas seal is prevented at shutdown by the combined sealing operation of the circumferential seal and the closed shutdown piston. When the compressor is restarted, the source of filtered higher pressure seal gas is returned, and the piston recesses to its normally open position.
The dry gas shutdown seal has an uncomplicated construction, and improves the performance, efficiency and longevity of the compressor.
These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.